HETEROBLASTIC DEVELOPMENT IN PLANTS
135
when the change from the juvenile to the adult stage was likely to have
occurred. There were also considerable differences in the wood from juvenile and adult regions of the tree. In the homoblastic species no corresponding differences were observed.
III. Interpretations and Experimental Studies
A. Phylogeny and Recapitulation
Before the introduction of experimental methods into the study of
heteroblastic development, most botanists accepted the "basic biogenetic
law" of Haeckel (1866), according to which ontogeny is a condensed
recapitulation of phylogeny. But the demonstration by Goebel (1896,
1908) and other early workers, notably Shull (1905), that the course of
heteroblastic development can be altered by changes in the environment,
and that in many plants reversion to juvenile or primary leaves can be
induced at will, cast considerable doubt on the importance of recapitulation in plant ontogeny. Few modern authors believe that ontogeny involves any wholesale recapitulation of ancestral adult forms. Nevertheless, since evolution can affect any stage of development, the retention or
indeed recapitulation of certain ancestral features would be expected during the ontogeny of any organisms in which evolutionary changes have
proceeded mainly by alterations in the terminal stages of development
(gerontomorphosis). On the hand, in organisms which have evolved
largely by the discard of later stages and the modification of earlier ones
(paedomorphosis), there can be only a very restricted opportunity for
such recapitulation. These aspects of ontogeny are discussed in greater
detail in the comparatively recent works of Takhtajan (1954), de Beer
(1958), Ewer (1960), and Smit (1962).
There is no evidence that the progressive changes observed during the
primary stages of development of most vascular plants represent any
recapitulation of ancestral forms, but some degree of recapitulation, in
the sense discussed above, is encountered both in many lower plants, and
in certain vascular plants, especially those with strongly modified adult
stages. Thus in the phyllode-forming species of Acacia, the seedlings bear
normal bipinnate foliage leaves, which are similar to the adult leaves of
the species without phyllodes. In this, and many similar examples, there
can be little doubt that the juvenile stage corresponds more closely to the
ancestral forms. Goebel (1898) believed that the pronounced differences
between certain juvenile and adult forms originated by adaptation of
the two stages to different environmental conditions. Even in a dry climate a seedling commences its growth at a time of relative abundance of
water, and only the adult stages are exposed to drier conditions. Similarly, in explanation of the frequent occurrence of strongly heteroblastic
135
when the change from the juvenile to the adult stage was likely to have
occurred. There were also considerable differences in the wood from juvenile and adult regions of the tree. In the homoblastic species no corresponding differences were observed.
III. Interpretations and Experimental Studies
A. Phylogeny and Recapitulation
Before the introduction of experimental methods into the study of
heteroblastic development, most botanists accepted the "basic biogenetic
law" of Haeckel (1866), according to which ontogeny is a condensed
recapitulation of phylogeny. But the demonstration by Goebel (1896,
1908) and other early workers, notably Shull (1905), that the course of
heteroblastic development can be altered by changes in the environment,
and that in many plants reversion to juvenile or primary leaves can be
induced at will, cast considerable doubt on the importance of recapitulation in plant ontogeny. Few modern authors believe that ontogeny involves any wholesale recapitulation of ancestral adult forms. Nevertheless, since evolution can affect any stage of development, the retention or
indeed recapitulation of certain ancestral features would be expected during the ontogeny of any organisms in which evolutionary changes have
proceeded mainly by alterations in the terminal stages of development
(gerontomorphosis). On the hand, in organisms which have evolved
largely by the discard of later stages and the modification of earlier ones
(paedomorphosis), there can be only a very restricted opportunity for
such recapitulation. These aspects of ontogeny are discussed in greater
detail in the comparatively recent works of Takhtajan (1954), de Beer
(1958), Ewer (1960), and Smit (1962).
There is no evidence that the progressive changes observed during the
primary stages of development of most vascular plants represent any
recapitulation of ancestral forms, but some degree of recapitulation, in
the sense discussed above, is encountered both in many lower plants, and
in certain vascular plants, especially those with strongly modified adult
stages. Thus in the phyllode-forming species of Acacia, the seedlings bear
normal bipinnate foliage leaves, which are similar to the adult leaves of
the species without phyllodes. In this, and many similar examples, there
can be little doubt that the juvenile stage corresponds more closely to the
ancestral forms. Goebel (1898) believed that the pronounced differences
between certain juvenile and adult forms originated by adaptation of
the two stages to different environmental conditions. Even in a dry climate a seedling commences its growth at a time of relative abundance of
water, and only the adult stages are exposed to drier conditions. Similarly, in explanation of the frequent occurrence of strongly heteroblastic